The limnological status of the Gwebi river, Zimbabwe

Abstract

The study aimed at assessing the limnological status of Gwebi River. Sampling was carried out from March to August 2015 on eight sites along Gwebi River. Five classes of land use/ land cover namely; woodland, grassland, built up water and bare land were considered. Total phosphorus, reactive phosphorus, total nitrogen, nitrate, ammonia, chemical oxygen demand, biological oxygen demand, pH, total dissolved solids, conductivity, salinity, dissolved oxygen, temperature and discharge were the measured variables. Macroinvertebrates were sampled according to the SASS5 protocol. River sediments were analysed for lead, cadmium and copper. Leaf litter decomposition was analysed from leaf packs and mass transportation of total nitrogen, BOD and total phosphorus were used to assess self purification of the river. There was an increase in the proportion of the area under woodland from 12.73 % to 22.39 % and on built up area from 1.46 % to 5.55% in the Gwebi River catchment area between 1998 and 2015. The physicochemical variables were significantly different among the sites (ANOVA, p<0.05). Site 1 (Pomona) had the highest salinity (0.2mgl-1), conductivity (798 μScm-1) and total dissolved solids (495mgl-1) compared to the other sites. Highest sediment lead (51.9mgl-1) and copper (35.8mgl-1) were also detected at Site 1. Site 5 (New Marlborough) and 6 (Old Mazowe) downstream of Marlborough sewage works had the highest concentrations of ammonia, total nitrogen and biological oxygen demand compared with the other sites. The South African Scoring System Version 5 (SASS5), Shannon-Wiener and Simpson’s diversity Indices were used to analyse the macroinvertebrate assemblages of Gwebi River. Sites 1 to 6 were classified as having fair water quality; Site 7 (AFDIS) and 8 (Nyabira) had good water quality basing on the average score per taxon (ASPT). Site 5 (New Marlborough) had the lowest Shannon-Wiener diversity (1.15) and Site 7 (AFDIS) had the highest diversity (2.16). Leaf litter decomposition was significantly different (ANOVA, p<0.05) across months. Decomposition was highest in May with an average weight loss of 5.79g per site compared to an average of 2.65g per site in June. Site 3 had the highest leaf mass loss due to the high numbers of shredders identified from the leaf packs. Site 5 had the lowest leaf mass loss from the leaf packs with an average weight loss of 2.5 g per month. Purification capacity of Gwebi River was analysed using the mass transport of total nitrogen, total phosphorus and biological oxygen demand. Stream purification was significantly different among the sites (ANOVA, p<0.05). Purification was most efficient between Sites 6 (Old Mazowe) and 7 (AFDIS). The influx of pollutants and sewage effluents exceeded the purification capacity at some sites. Sewage effluent from Marlborough as well as pollutants from farmland in the river catchment area had a negative effect on water quality of Gwebi River.